Booting and Halting Chapter 4. Chapter Objectives Examine the sequence of events that occur when a system is booted. Examine the methods used to modify.

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Presentation on theme: "Booting and Halting Chapter 4. Chapter Objectives Examine the sequence of events that occur when a system is booted. Examine the methods used to modify."— Presentation transcript:

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Chapter Objectives Examine the sequence of events that occur when a system is booted. Examine the methods used to modify the boot sequence. Examine how to use the PROM monitor to select alternate boot devices. Examine the boot manager and boot loader. Examine how to shut a system down properly.

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An Overview Of The Boot Sequence When power is applied, the system resets all of the logic gates (chips) to a known state. A series of diagnostic tests (Power-On Self- Test, or POST) are run from the PROM Monitor to ensure that some minimal set of hardware components are available for system operation. Once the POST operation completes, the system loads a small “boot loader” program from the system PROM. The boot-loader loads the system boot block.

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An Overview Of The Boot Sequence The system executes the code from the boot block. –This program loads the secondary boot loader that will eventually load the operating system kernel and start it. Most secondary boot loaders allow the operator to select the device to boot from, as well as the program that will be loaded into memory and started.

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An Overview Of The Boot Sequence The kernel creates a few necessary processes: –the init process, –a process scheduler, –and usually a few memory management processes. The init process runs a series of start-up scripts that configure the system for multi-user operation. Now that we’ve see the basic series of events that occur during the system boot process, let’s examine some of the internal events that occur during each major step of the boot process.

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Step 1: PROM Monitor The PROM monitor includes a small program (Power On Self Test, or POST) that initializes the system hardware, and runs a miniature diagnostic program. –The diagnostics ensure a minimally operational base for the OS to run on. –POST diagnostic routines do not guarantee that the system hardware is fully functional. Once the diagnostic completes, the PROM monitor probes the system busses to determine what hardware is connected to the system. Once the hardware has been probed, the PROM monitor loads and starts the second phase of the boot process; the boot block.

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Step 2: Boot Block The boot block’s job is to initialize some of the system’s peripherals and memory, and to load yet another program, the secondary boot loader, (sometimes known as the boot manager) that will load the OS kernel. A boot block is typically placed on the disk as part of the OS installation process. The sysadmin should be familiar with the similarities and differences between the following boot blocks: –Windows Boot Block –Linux Boot Block –Solaris Boot Block –IRIX Boot Block –MacOS and AIX Boot Block

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Step 4: The OS Kernel –Once the kernel is loaded, and started, it probes devices. Some architectures use the PROM monitor to build a device tree that is handed to the kernel during the boot process. Other architectures require the kernel to probe the system busses for devices to build the device tree. –Once the device tree is available, the kernel parses it, and each device is probed to determine if it is operational, (and if so, the driver module is loaded into the kernel). This search for memory and devices is sometimes referred to as auto-configuration.

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Step 4: The OS Kernel UNIX Run Levels –All flavors of UNIX, and UNIX- work-alikes use similar foundations for the system run modes. –As far as UNIX and its ilk are concerned, there are basically two run modes: single user (sometimes called maintenance mode), and multi-user. –There may be several forms of the multi-user mode (with services, without services, and so on) on any given UNIX OS.

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Step 4: The OS Kernel MacOS Run Levels –MacOS also provides for “multiple” run levels. On the older versions of MacOS, different run levels meant simply a choice between “boot normally”, or “boot without extensions”.” Now that MacOS X is a Mach/UNIX based kernel, there are more run levels available. –The MacOS X kernel enjoys the same set of run levels, as do the other UNIX variants.

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Windows Run Levels Windows has a limited set of run levels –Multi-user –Safe Mode –Safe mode with networking Typically used to repair damaged system.

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Step 4: The OS Kernel Start-up Scripts –(Un)fortunately, within the OS world there are several “camps” of believers when it comes to how the system should start system services. –The following sections provide an overview of the details of the Windows, MacOS, System V UNIX, and BSD start-up sequences, pointing out OS- specific oddities as applicable.

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Step 4: The OS Kernel Typically, BSD variants include a run control (also known as an rc) script in the /etc directory. –This allows the administrator to edit a single /etc/rc script to make it start a new service at boot time. Other BSD variants have simply increased the number of rc files in the /etc directory. –For example, the FreeBSD start-up directory contains scripts with names such as /etc/rc.atm, /etc/rc.firewall, and /etc/rc.network. These individual scripts, respectively, configure the ATM network cards, cause the system to become a firewall, and configure the network links respectively. These scripts are called by the master script, /etc/rc

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rc script problems Using a small number of rc scripts can cause problems: –Startup becomes more difficult to test, as each script starts multiple services. Need to reboot system to test scripts. –Not easy to “shut down” individual services using rc scripts.

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System V Scripts System V employs a directory full of init scripts. Each System V variant seems to put these scripts in a different directory: –Solaris /etc/init.d –Linux /etc/rc.d/init.d –HPUX /sbin/init.d Each script starts/stops one service. Symbolic, or hard links connect the /etc/init.d scripts into the correct run level directories (/etc/rcN.d/ where “N” is the run level between 0 and 6). As system enters a run level, the scripts in the corresponding run level directory are executed: –First the K (Kill) scripts are run in numeric, then alphabetic order. –Next, the S (Start) scripts are run in numeric, then alphabetic order.

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Benefits of init Scripts One of the benefits of using init directory scripts is that they are easily tested. –The scripts may be manually invoked with the stop and start arguments as a check to determine whether they function correctly before creating the links to the rc directories, and trying them under actual system boot conditions. –This procedure is recommended because it can help you catch mistakes that might interrupt the boot process and leave the system unusable.

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Summary The administrator should understand the sequence of events that occur when a system is booted. The administrator should understand the methods used to modify the boot sequence. The administrator should know how to use the PROM monitor to select alternate boot devices. The administrator should understand the workings of the boot manager and boot loader. The administrator should understand how to shut a system down properly.